The present invention generally relates to apparatus and methods related to a journal foil bearing. More specifically, to a journal foil bearing comprising multiple foils, wherein each may have a different radius of curvature.
Fluid film hydrodynamic bearings may operate on the principle that a high speed rotating member, such as a shaft, is at least slightly eccentric with respect to rotation about its longitudinal axis. Therefore, if the shaft is enclosed by a close-fitting, compliant, annular element such as a thin foil encased within a stationary retaining member, the eccentricity of rotation within such retaining member will form and maintain a pressurized fluid (e.g., air) film between the shaft and the compliant foil. The high speed rotation of the shaft generates a high pressure in the fluid film, which fluid film supports the load imposed by the shaft. The lowest rotational speed at which this occurs is known as the lift-off speed. A spring foil, i.e., a resilient backing member, may be disposed between the compliant foil and the stationary member (sometimes referred to as a cartridge, retainer or base) to accommodate deflections of the foil resulting from pressurization, centrifugal forces and temperature differentials in order to maintain optimum or at least adequate film layer geometry. The fluid film hydrodynamic bearing desirably has high load capacity and high damping for suppression of shaft whirl.
Rotor speeds for machines that use foil bearings may sometimes be limited by the amount of damping that the foil bearings provide. High damping is required to suppress the shaft whirl increasing to a point where it can become unstable. Increasing the pre-load on the shaft, which increases the amount of resistance to turning imposed on the shaft by the foil bearings prior to the shaft reaching the lift-off speed, can increase the coulomb damping, as the friction of coefficient of the foils (of the foil bearing) sliding against each other is proportional to the preload on the shaft. However, higher pre-load is not desired, since it may also increase the starting torque, accelerate the wear of a coating on the foil, and may increase the lift-off speed.
It may be desirable to provide an increased amount of damping while decreasing the pre-load, as by increasing damping, the stability of the rotor at high speeds will be improved. By decreasing pre-load, a lower starting torque will be required, which may improve the durability of the bearing foil.
Examples in the art directed towards an increased amount of damping include U.S. Pat. No. 5,634,723 (Agrawal), which is directed to a design which attempts to increase the coulomb damping of the foil bearing by anchoring different layers of a foil bearing, namely the top foil and the spring foil, in opposite directions to facilitate sliding of the foil layers relative to each other. This approach does not however address increasing fluid film or squeeze film damping, while decreasing bearing pre-load.
U.S. Pat. No. 4,262,975 (Heshmat) is directed to a foil bearing having a variable-pitch spring that becomes stiffer in the direction of the shaft rotation (i.e., from a leading edge of the foil to a trailing edge of the foil). This design attempts to create damping from the relative motion of the layers of foils above the spring, once again focusing on the coulomb damping, but not addressing fluid film damping nor preload on the bearing.
U.S. Pat. No. 4,277,113 (Heshmat) is directed to a foil bearing having a composite material top foil comprising a layer of copper diffused into a steel alloy foil material. This design attempts to increase coulomb damping from the higher friction coefficient of the copper relative to the steel alloy, and from the viscous damping provided by local yielding of the copper layer. This approach also does not address bearing preload, and requires a complex bearing foil having limited durability.
As can be seen, there is a need for a journal foil bearing which provides increased fluid film damping to rotating shafts, while also providing the improvements in performance and durability associated with bearings having relatively lower bearing preload.